High pressure metal vapor lamp



Inventor: ydney AR. Rigden,

M His Atto ney Jan. 28, 1958 s. A. R. RIGDEN HIGH PRESSURE METAL VAPORLAMP Filed July 15, 1952 S YM -K United States Patent HIGH PRESSUREMETAL VAPOR LAMP Sydney A. R. Rigden, London, England, assignor togeneral Electric Company, a corporation of New ork Application July 16,1952, Serial No. 299,127

Claims priority, application Great Britain August 9, 1951 1 Claim. (Cl.313-484) This invention relates to high pressure metal vapor electricdischarge lamps comprising within a sealed envelope at least one pair ofmain electrodes for the passage of the lamp discharge, and a filling ofvaporizable metal together with a small quantity of rare gas such asargon for facilitating starting of the discharge. The metal of thefilling is usually mercury, with which may sometimes be included one ormore other metals, such as cadmium or zinc, for modifying the color ofthe light from the discharge; the pressure of the rare gas components ofthe filling is usually about 30 mms. rise of mercury column. The metalvapor pressure, during operation, is usually in excess of about a halfatmosphere and may be many atmospheres.

For starting such lamps, it is usual to apply an overvoltage, greaterthan the normal operating voltage of the lamp, between the mainelectrodes of the lamp, or between one main electrode and an auxiliaryelectrode mounted adjacent to this main electrode, but difliculties havebeen experienced due to the magnitude of this overvoltage required tostart the lamp being inconveniently high with regard to the normal lampoperating voltage.

The object of this invention is therefore to provide an improved form oflamp with which the magnitude of the over-voltage required for startingthe discharge is reduced as compared with similar known lamps.

According to the invention, in a high pressure metal vapor electricdischarge lamp of the type specified, the inner end of at least one mainelectrode is provided by a hollow block of tantalum containingactivating material and pierced by one or more passages through whichthe activating material can diffuse to the terminal surface of theelectrode.

The term inner end means of course that end of the electrode on whichthe discharge terminates in full normal operation, and the terminalsurface of the electrode means the part of the end on which thedischarge actually does terminate. The activating material may consistof any suitable refractory electron emissive material, for examplesintered alkaline earth oxides, or thoria.

I have found that lamps in accordance with the invention, after theinitial period of operation usually referred to as ageing, giveuniformly low starting voltages, lower than have previously beenobtained with known lamps of the type specified, not using tantalumelectrodes.

I believe this to be due to the fact that the tantalum inner end of thesaid main electrode acts as a selective getter during the initial ageingof the lamp, absorbing gaseous impurities without appreciably affectingthe metal or rare gas filling, and thereby preventing these impuritiesfrom giving rise to an increase in the starting voltage such as haspreviously been experienced with known lamps of the type specified. Theactivating material within the hollow end of the electrode ensures thatan adequate electron emission is obtained from the electrode inoperation of the lamp.

The said main electrode may consist entirely of a tantalum block such asa rod, suitably hollowed at its ice i on

inner end, or the hollow tantalum inner end may be attached to a part ofthe electrode which is of a different metal, for example, tungsten ormolybdenum; the hollow tantalum block may be attached to a part of theelectrode which is of a different metal by means of cooperating screwthreads formed on the said electrode and on the tantalum block.

In the case of a lamp designed for operation on an alternating supply,preferably each main electrode is provided with a hollow tantalum innerend in accordance with the invention.

One embodiment of the invention will be described, by way of example,with reference to Fig. 1 and Fig. 2 of the accompanying drawings,wherein:

Fig. 1 shows a perspective view of a high pressure metal vapor electricdischarge lamp in accordance with the invention, having a mid-portion ofthe lamp broken away for clearness, and designed for operation from analternating supply and;

Fig. 2 shows a cross-sectional view along the longitudinal axis of oneof the lamp electrodes.

Referring now to Fig. 1, the lamp shown is a high pressure mercurycadmium filled compact source lamp adapted to dissipate 2.5 kilowatts innormal operation and comprises a spheroidal quartz bulb 1 of longerinternal diameter 61 mms. and shorter internal diameter 56 mms.,provided at opposite ends of its longer diameter with tubular sealingstems 2, 3 through which pass molybdenum rods 4, 5 respectively, each ofdiameter 4 mms., the sealing stems being closed at their outer ends bylead-in seals 6, 7 respectively; the bulb is provided with a filling ofargon gas at a pressure of 30 mms. of mercury and a globule of 0.47 gm.of mercury together with 0.47 gm. of cadmium.

Each of the molybdenum rods 4, 5 is provided at its inner end 8, 9respectively, with a hollow cylindrical tantalum block 10, 11respectively, having an external diameter of 6 mms., an internaldiameter of 3.5 mms., and an over-all length of 15 mms.; the terminalsurfaces 12, 13 of the blocks Ill, 11 respectively, are spaced l5 mms.apart and each surface is provided, as shown in greater detail in Fig.2, with a hole 14, 15 respectively, having a diameter of 1 mm. which isarranged to pass through the block between the terminal surface 12 andthe inner surface 12' of the block; the distance between the surface 12and the inner surface 12' of the block is 2.5 mms.

For mounting each of the blocks 10', 11 on the molybdenum rods 4, 5respectively, each of the rods is provided at its end 8, 9 respectivelywith a screw thread, one of which, 16, is shown in Fig. 2 forcooperating with a similar screw thread, one of which, 17, is shown inFig. 2, formed in the internal surface 18 of the block it). Pellets, oneof which 19 is shown, containing activating material comprising amixture in the indicated proportions, by molecular weight, of bariumoxide-2.5, calcium oxide 1.0, thorium oxide3.0, and having a quantity oftungsten powder, of a grain size such that it will pass through a 200mesh sieve, and of weight equivalent to 3% of the total weight of theabove mixture of oxides, added to this mixture, are contained within thehollow blocks 10, 11 and held in position within the blocks by the rods4, 5.

For reducing blackening of the envelope 1 of the lamp, in operation, dueto the are possibly wandering over the surface of the tantalum block sothat it comes into contact wtih the molybdenum rods 4, 5 respectively,thereby causing molybdenum to be vaporized onto the interior surface 20of the envelope 1, tungsten coils 21, 22 respectively are provided onthe rods so that the ends 23, 24 of the coils abut the adjacent ends ofthe blocks 10, 11 respectively, and the other ends 23', 24' of the coils3 abut the adjacent ends of the sealing stems 2, 3 respectively.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

A high pressure metal vapor lamp operable at kilowatt loadingscomprising a sealed envelope containing an ionizable medium including aninert starting gas and vaporizable metal including mercury in an amountsufficient to furnish a pressure in the range of approximately 1atmosphere and above during operation of the lamp, a pair of mainoperating electrodes sealed into said envelope and having relativelyclosely spaced conjugate terminal surfaces defining a short are gap, atleast one of said electrodes comprising a molybdenum rod having fastenedto its inner end a relatively massive hollow block of tantalumcontaining in the interior thereof activating material comprising amixture of approximately 2.5 parts barium oxide,

1 part calcium oxide and 3 parts thorium oxide by molecular weight and atungsten powder in an amount of about 3 percent by weight of the saidoxides, said tantalum block being provided with an. aperture through itster minal surface exposing the activating material within it andallowing diffusion of activating material to its terminal surfaces, anda coil of tungsten wire wound around said molybdenum rod and extendingup to said tantalum block.

References Cited in the file of this patent UNITED STATES PATENTS2,175,345 Gaidies Oct. 10, 1939 2,249,672 Spanner July 15, 19412,453,118 Buckingham Nov. 9, 1948 2,460,738 Francis Feb. 1, 1949

